Communication system



July 6 1943- R. H. CAMPBELL COMMUNICATION SYSTEM 2 Sheets-Sheet 1 Filed July 1, 1940 July 6, i943. R. H. CAMPBELL COMMUNICATION SYSTEM Filed `Ju1y l, 1940 2 Sheets-Sheet 2 Patented July 6, 1943 2,323,307 COMMUNICATION SYSTEM Richard H. Campbell. Racine, Wis., assignor to Webster Electric Company, Racine, Wis., a corporation of Delaware Application July 1, 1940, Serial No. 343,390

25 Claims.

The present invention relates to intercommunicatlon systems, and particularly to intercommunication systems of the loud-speaking type wherein a number of stations Vcomprising the system are provided with a loudspeaker-type receiver, a microphone, and an amplifier so constructed and arranged with respect to each other n that two-way communication -over separate interstation circuits may be carried on without manipulation of switches.

In systems of this type, difficulties have been encountered because of excessive feedback, i. e., feedback causing continuous audio oscillation resulting from the proximity of the receivers and microphones at the respective stations, there being a complete: electro-acoustical circuit whenever two or more stations are connected for communication. The electro-acoustical circuit includes the acoustical circuits between the microphones and receivers at each station and electrical circuits each of which includes a microphone and amplifier at one station, the interstation communication circuits and the receiver at the other station. It is an object of the present invention t provide a system of this type in which these difliculties are overcome.

It is a further object of the present invention to provide an arrangement whereby two-way communication can be carried on at a predetermined volume or sound level and in which one station may call another with greater volume, or in which communication may be carried on at high volume alternately in opposite directions.

A further object of the invention is to provide a system of the type last mentioned, wherein excessive feedback is avoided even when two stations simultaneously call each other. This is accomplished by arranging and constructing the calling and volume control means so that the sound level remains at its original value when two or more stations simultaneously call each other, or attempt to communicate with each other at the higher sound level.

A further object of the invention is to provide a unit comprising 'a microphone, loudspeaker, and amplifier with testing and adjusting means, whereby it may be readily tested and adjusted for use in a system comprising a number of similar units. t

A further object of the present invention is the provision of a method of adjustingk units of the type described, which comprises connecting the output of an amplifier at Aone station to the input of the loudspeakerat that station, and adjusting the amplification to a point just below that at which excessive feedback occurs and then connecting the output of the amplifier to the loudspeaker and operating the high volume calling means and adjusting the volume of the speaker to a poi-nt just below that' at which excessive feedback occurs.

A further object of the present invention is to provide a unit of the type described with amplication increasing means and reception volume decreasing means so constructed and arranged that the increase is in the same relation as the decrease to prevent excessive feedback.

Other objects and advantages of the present invention will become apparent in the course of the ensuing description, in which reference is had to the accompanying drawings in which:

Fig. 1 is a schematic representation of a system embodying the present invention;

Fig. 2 is a parti-a1 schematic diagram showing the connections of-a microphone of the crystal type to the input of the amplier;

Fig. 3 is a schematic representation of a modifled form of system embodying the present invention; and

Fig. 4 is a partial schematic representation of a further modification of the invention.

Referring first to Fig. 1, it may be seen that reference characters l to 4, inclusive, indicate a plurality of stations of a multiple station intercommunication system of the loudspeaking type.

The stations are located remote from each other.

and from other stations of the system, which have been omitted for the sake of simplicity and brevity, but which stations may be provided with apparatus similar to that illustrated.

The present invention relates primarily to systems wherein a plurality of stations are each provided with an amplier, a. microphone and loudspeaker-type receiver, and in which the microphone is connected directly to the input of the amplifier and a loudspeaker is supplied with amplified signal current from another station. Thus, in the embodiment illustrated in Fig. 1, stations l and 2 are provided with microphones I0 and I2, loudspeakers I4 and I6, and amplifiers I8 and 20, respectively. Stations 3 and 4 have been illustrated as being provided only with loudspeakers 22 and 24, respectively, for the purpose of better illustrating certain advantages of the invention. In order to cut down the amount of feed back from the loudspeakers to the microphones,v these elements are located away from each other-,as far as convenient.. Forinstanca the microphones may be placed on top vof the usual cabinetin speaker at the front thereof in accordance with usual practice. In order to obtain the highest possible volume without excessive feed back the amplifiers. microphones and loudspeakers are made with as fiat a frequency response as possible.

The amplifiers I8 and 20 are identical in construction and consequently only the amplifier at station I has been illustrated completely. The microphone I is most conveniently, but not necessarily, connected directly to the first tube 28 (a 6F5 tube) of the amplifier, which is a threestage resistance coupled amplifier. The second stage of amplification is provided by a tube 28 (also a 6F5 tube), and the third stage by a tube 88 (a GF6 tube).

The microphone I0 is connected directly to the grid of tube 26 by a pair of conductors 82, 84, the latter of which may be, and is illustrated as being, connected to ground (which may be the grounded chassis of the amplifier), as indicated by the reference character 88. The microphone is preferably ofthe crystal type, as illustrated in greater detail in Fig. 2. Referring now to this figure, the crystal is indicated by reference character 88. It is shunted by a high resistor 40 having a resistance of about five megohms. The conductor 32 leading to the grid of tube 26 is provided with a shield 42, illustrated diagrammatically and connected to the ground conductor 34 by a conductor 44.

Returning now to Fig. l, it may be seen that the tubes`26, 28 and 30 are each provided with a grid biasing resistor 46 shurited by a by-pass condenser 48. The resistors associated with tubes 26 and 28 have resistances of 2700 ohms and that associated with tube has a resistance of 470 ohms. The condensers 48 have a capacity of 10 mlcrofarads each. The tubes 26 and 28 have .004 microfarad blocking condensers .50 in their output circuits, and tube 26 has its output connected across a one megohm coupling resistor 52, while tube 28 has its output connected across a combined coupling and potentiometer .5 megohm resistor 54 having associated therewith a movable contact 55 for a purpose to be described shortly. The output circuit of tube 28 is also provided' with a one megohm volume control resistor 56 provided for the purpose and controlled in a manner hereinafter to be described in greater detail.

'I'he amplifier is supplied with power from a suitable alternating, current source through a transformer 51 and a full-wave rectifier 58. Power is supplied to the primary of the transformer 51 through a pair of conductors 60, under the control of a switch 62, which may be a part of a volume control switch to be described later. The power supply circuit may be of any wellknown type, and it is believed sufiicient for the purpose of this specification to state that it comprises the usual filter 64 and that suitable plate potential is supplied to tubes 26, 281 and 80 through one-fourth megohm resistors 66 and 68 and through the primary winding 101 of the output transformer 12 in the output circuit of tube 30, respectively.

The amplifier output may be connected to related stations remote from station I by means of a plurality of selector switches 14, 16 and 18 connected to the secondary winding 810 of the output transformer 12 through a pair of conductors 82.

Before passing from the description ofthe amplifier per se, it may be well perhaps to state that the tubes 26, 28 and 30 are of the indirectly heated type, the cathode heaters of which are supplied with current from a separate secondary winding 84 provided on the transformer 51. A pilot light 86 is connected across a portion of this secondary winding to indicate the operative condition of the amplifier.-

As previously indicated, the loudspeakers at the various stations are adapted to" be supplied with amplified voice currents from the other stations. Thus, the loudspeaker I4 at station I is connected to a pair of conductors 88, which may be termed the station conductors. Conductors 88 terminate in a manually operable volume control, indicated generally by reference character 80, and which may comprise a knob located outside the cabinet in which the unit is usually mounted. Loudspeaker |4 is normally connected across a portion of the volume control .resistance by conductors 92 and 94, the former of which has inserted therein an adjustable resistor 96, which is termed a high volume feedback adjustment.

vReceiver I4 is normally so connected in circuit as to receive calls at high volume by means comprising high volume switch |00 having a blade |02 normally short circuiting the resistor 86. The switch is preferably of either the push button or key type and is biased into the position in which it is indicated.

The amplifier |8 normally transmits signals' from station I to other stations at low volume by reason of the fact that resistor 56 is inserted in the output circuit of the second stage amplifier tube 28.

The highvolume switch |00 is provided for the purpose of enabling the operator at station I to call another station at high volume. It comprises the previously-mentioned switch blade |02 which controls resistor 96 and a second movable switch blade |04 which controls the connection of resistor 56. When the switch |00 is in its low volume position, the resistor 96 is short circuited and resistor 56 is inserted in the output circuit of tube 28 and in series with a portion of resistor 54, thereby conditioning the receiver for reception at high volume and the amplifier for transmission at low volume. The resistor 96 is short circuited through a circuit including conductors |06 and |08. When the switch is operated to its displaced position, resistors 96 and I4 are placed in series and parallel, respectively, with the loudspeaker |4, and resistor 56 is short circuited through conductors ||0 and ||2. 'I'he loudspeaker may have a resistance of about 45 ohms and resistors 96 and 4 may have resistances of about 350 and 25 ohms.

The switch blades |02 and |04 of the high volume switch should be so arranged with respect to their associated contacts that the resistor ||4 is connected in circuit before the resistor 56 is short circuited and is disconnected after the short circuit is removed from across the latter, in order to make certain that the amplifier gain is not increased before the reception volume is decreased or that the reception volume is not increased before the amplifier gain is decreased.

In order to effect the adjustment of the apparatus at station so that excessive feedback will not occur during operation, there is provided a test feedback switch |20 by means of which the output of the amplifier may be connected directly to the loudspeaker circuit. 'I'he switch |20 normally occupies a position in which the output of the amplifier is disconnected from the receiver circuit and is operable to a displaced position to connect the two circuits. It comprises a pair of movable switch blades |22 and a pair of contacts |24l thc blades |22 being connected by conductors |26 across the interstation conductors 88 and the contacts I 24 being connected by conductors |28 directly across the output conductors 82, The test feedback switch may be eliminated if desired, as the amplifier output may be connected to the receiver by other means, as through the output and receiver conductors.

To adjust the unit at station I for operation in a system comprising a number of similar units, the output of the amplifier is connected to the receiver I4, with the volume control 90 adjusted to its maximum volume position, by depressing the test feedback switch |20 and adjusting the movable contact 55 associated with the volume control resistor 54 to a point just below that at which the feedback from the output to the input is sufllcient to cause the amplifier to go into an oscillatory condition (i. e., to go into a condition in which continuous audio oscillation occurs), and then by depressing both the high volume switch |00 and the test feedback switch |20 and adjusting resistor 96 to a point just below that at which the feedback is sufficient to cause the amplier to go into an oscillatory condition. The adjustment of contact 55 is preferably such that it cannot be made simply by an operator. It may be. for example, accessible only with difiiculty and adjustable only by means of a special tool such as a screw driver.

It may be seen, therefore, that the two adjustrnents result in the adjustment of the arnplier so that excessive feedback does not occur when the unit is conditioned for reception at low volume and transmission at high volume, or for transmission at loW volume and reception at high volume, Likewise, by making the adjustments with manual volume control 90 at its maximum volume position, an operator is prevented from adjusting the volume so as to create a condition wherein excessive feedback obtains.

The apparatus at station 2 corresponds with that at station I. The receiver I6 is connected directly to interstation conductors |30 (the conductors individual to station 2) and the output circuit of amplifier I8, comprising conductors |32. is adapted to be connected to various interstation conductors, including the conductors 88 individual to station I, by means of normally open selector switches |34, |36 and |38. Switches |36 and |38 are adapted, as are the switches 16 and 18 at station I, to connect the output of the amplifier to interstation conductors |40 and |42 associated with receivers 22 and 24 at stations 3 and 4, respectively.

The interstation conductors |30 individual to station 2 are connected across a manually operable volume control |44, from which a pair of conductors |46 and |48 lead to the loudspeaker I6. The former has in circuit therewith an adjustable resistor |50 corresponding to resistor 95 at station I. Resistor |50, as well as the resistor 56 associated with the amplifier I6, which corresponds to resistor 56 at station I, are controlled by a, high volume switch |52 normally short circuiting resistor |50 through conductors |54 and |56 by a switch blade |58, and adapted when depressed to a second position to short circuit resistor 56 through conductors |60 and |62 by a switch blade |64 and to place a resistor |64 in parallel with the loudspeaker I6,

Station 2 is also provided Awith a test feedback switch. indicated generally by reference character |66. It comprises a pair of simultaneously movable switch blades |68 and a pair of contacts |10. The switch is normally in circuit opening position, but is adapted to Abe depressed to connect the output of the amplifier to the receiver I6, through a first pair of conductors |12, connected across the volume control |44 and a second pair of conductors |14 connected across the output conductors |32.

The apparatus at station 2 is adjusted for operation in the same manner as that at station so that it is not deemed necessary to repeat the description of the mode of adjustment.

The above described adjustments of the units may be eiected at the factory or at the place of installation, or for that matter at both, so that it is a relatively simple matter to provide a system comprising any number of stations in which excessive feedback does not occur.

In the description of the operation, it is assumed rst that the operator at sfation I desires to call an operator at static-n 2. The operator at station I depresses selector switch 18 to connect the output of amplifier I8 directly to the loudspeaker I6, which is conditioned for high volume reception because of the position of switch |52- The operator at station may call the operator at staticn 2 at low volume simply by energizing the amplifier I4 by operating the combined volume and power switch 62, and talking into the microphone. If the operator at station I knows that station 2 is located at a relatively noisy place, or for any other reason desires to call the operator at station 2 at high volume, he can do so by depressing the high volume switch |00 from its normal to its displaced position. Under normal conditions there is no danger of feedback at this time, because the amplifier at station 2 is inoperative so that there is no feedback path.

When the operator at station 2 responds to the call, he closes selector switch |34, thereby connecting the output from the amplifier 20 directly to the receiver I4 at station and connects the amplier to its source of power. The two, operators can talk to each other at low Volume and excessive feedback will not take place, because of the previously described adjustment of the units at both stations. If the operator at station 2 has diiliculty in hearing, he may notify the operator at station of this fact, and the Ylatter needs only to depress the high volume switch to boost the amplifier gain and decrease the listening Volume at station I. As a result of this, the operator at station 2 is better able to hear the operator at station I. The high Volume switch boosts talking volume and decreases listening volume in the same relationship so that excessive feedback does not take place.

If both of the stations are located in relatively noisy localities, then high volume alternate twoway communication'can be carried on by the operators alternately depressing their high volume switches when they talk and releasing them when they listen.

If the operators at both stations simultaneously depress their high volume switches, as when they call each other simultaneously, or when they are talking in alternate directions at high volume, thevolume at each unit stays the same as if the high volume switches were not depressed. This results from the fact that while the amplification at each station is boosted, the reception is decreased in the same relationship so that the volume remains the same.

It may be mentioned at this point that the stations, or one of them, one or more of the switches 1,6 and 18 or |36 and |38 are operated, in order to connect the output oi' the desired amplifier to the selected station or stations. Then, by depressing the high volume switch or |62, as the case may be, these stations may be called at high volume.

Another arrangement for effecting the advantageous results obtained with the above described embodiment is illustrated in Fig. 3,'to which reference will now be had. In this embodiment certain of the stations are provided with amplifiers of the inverse feedback type to provide a system having the numerous advantages brought out hereinafter.

The reference characters to 4, inclusive, are again utilized to indicate certain of the stations of a multiple station intercommunicating system of the loudspeaking type. The stations are located remote from each other and from other stations of the system, which have been omitted for the sake of simplicity and brevity, but which stations may be provided with apparatus similar to that illustrated.

Stations and 2 are provided with microphones l0 and i2, loudspeakers i4 and I6, and amplifiers 202 and 204, respectively. Stations 3 and 4 have been illustrated as provided only with loudspeakers 22 and 24, respectively, for the purpose of better illustrating certain advantages of the invention. As in the previously described embodiment, the microphones and loudspeakers at the respective stations are located away from each other as far as convenient in order to out down the amount of feedback between them and these elements as Well as the amplifiers are made with as flat a frequency response as possible, to obtain the highest possible volume before excessive feedback occurs.

Ampliiiers 202 and 204 are identical in construction and consequently only the amplier at station has been illustrated fully. The microphone |0 is connected directly to the first tube 206 (a 6F5 tube) of the amplifier, which is a three-stage resistance coupled amplifier provided with inverse feedback. The second stage of amplification is provided by a tube 208 (a 6SF5 tube), and the third stage by a tube 2|0 (a 6V6 tube).

The microphone |0 is connected directly to the grid of tube 206 by a pair of conductors 2|2 and 2|4, the latter of which may be and is illustrated as being connected to ground (which may be the grounded chassis of the amplifier), as indicated by reference character 2|6.

The tubes are each provided with a grid biasing resistor 2|8 shunted by a by-pass condenser 220. The resistors associated with tubes 206, 208 and 2|0 may have resistances of 6,000, 5,000 and 260 ohms, respectively, and the condensers a capacity of 10 microfarads each. The tubes 206 and 208 have .004 microfarad blocking condensers 222 in their output circuits, and tube 206 has its output connected across a one megohm coupling resistor 224 and a combined coupling and feedback control resistor 226. having associated therewith a movable contact 228 connected to a feedback conductor 230. The resistors 224 and 226 may have resistances of about 1 megohm and 25,000 ohms, respectively. The output circuit of tube 206 is provided with a .5 megohm coupling resistor 232.

The amplifier is supplied with power from a suitable alternating current source through a transformer 234 and a full-wave rectifier 266. Power is supplied to the primary of the transformer 234 through a pair of conductors 236 under the control of a switch 240. which may be a part of a volume control switch to be described hereinafter. The power supply circuit may be of any well-known type. and it is believed sumcient for the purpose of this specification to state that it comprises the usual filter 242 and that suitable plate potential is supplied to tubes 206, 208 and 2|0 through a .5 megohm resistor 244, a .25 megohm resistor 246, and through the primary winding 248 of the output transformer 250 in the output circuit of tube 2|0, respectively.

The output transformer 250 has associated therewith a pair of secondary windings 252 and 254, the former of which is connected through conductors 256 to station selector switches 258. 260 and 262, and the latter of which is connected to ground by a conductor 264 and to the feedback conductor 230 through a conductor 266 and a resistor 268 which may have a resistance of 10,000 ohms.

'I'he amplifier tubes are of the indirectly heated cathode `type, the cathode heaters of which are supplied with current from a separate secondary winding 269 on transformer 234. A pilot light 210 is connected across a. portion of this secondary winding to indicate the operative condition of the amplier.

The loudspeakers at the various stations are adapted to be supplied with amplified voice currents from the other stations. Thus, the loudspeaker |4 at station isconnected to a pair of conductors 212, which may be termed the station I conductors. These conductors terminate in a manually operated volume control, indicated generally by reference character 214, and which may comprise a knob located outside of the cabinet in winch the unit is usuali; mounted. Loudspeaker i4 is normally connected across a portion of the volume control resistance by conductors 216 and 218, the former of which has in'- serted therein an adjustable high volume feedback resistor 280.

Loudspeaker I4 is normally connected in circuit to receive calls at high volume by means comprising the high volume switch 282 having a blade 284 normally short circuiting the resistor 280 through conductors 285 and 286. The switch is preferably of either the push button or key type and is biased into the position in which it is indicated.

The amplifier 202 normally transmits signals from station to the other stations at low volume by reason of the fact that the inverse feedback circuit, which includes the previously mentioned secondary winding 254 and resistor 268, is normally effective to cut down the gain of the amplifier. When it is desired to operate the amplifier at high volume, the high volume switch 282 is operated to its displaced position, at lwhich time the previously mentioned switch blade 284 is effective to place the resistor 280 in series 'with the loudspeaker and to connect resistor 281 in parallel with the loudspeaker, and which is effective also to ground the feedback conductor 230 through conductor 286, switch blade v290, and a grounded conductor 292. It may be seen, therefore, that the operation of the high volume switch 282 results not only in increasing the gain of the amplifier, but also in decreasing the loudspeaker reception volume. The circuit constants are so adjusted that the increase in gain and the decrease in reception volume are substantially equal so that excessive feedback does not take place. In the instant embodiment the loudspeaker may have a resistance of 45 ohms and the resistors 280 and 281 may have resistances ofv 35.0 and 25 ohms, respectively.

The arrangement of the blades and contacts of the high volume switch should be such that the reception volume is decreased before the aniplier gain is increased and increased after the amplifier gain is decreased, just as in the case of the embodiment of Fig. 1.

In order to eect the adjustment of the apparatus at station I so that excessive feedback does not occur during operation, there is provided a test. feedback switch 294 by means of which the output of the amplifier may be connected directly to the loudspeaker circuit. The switch normally occupies a position in which the output of the amplifier is disconnected from the loudspeaker circuit but is operable to a displaced position to connect the two circuits. It comprises a pair o f movable switch blades 296 and a pair of contacts 298 connected to the receiver and amplifier output circuits by the pairs of conductors 300 and 302, respectively.

To adjust the unit at station I for operation in a system comprising a number of similar units, the output of the amplifier is connectedl to the loudspeaker I4, with the volume control 214 adjusted to its maximum volume position, by depressing the test feedback switch 294 and then adjusting the movable contact 228 associated with the feedback and volume control resistor 226 to a point just below that at which the feedback from the output to the input is sumcient to cause the ampliiier to go into an oscillatory condition, and then by depressing both the high volume switch 282 and the test feedback switch 294 and adjusting resistor 280 to a point just below that at which the feedback is suflicient to cause the amplifier to go into an oscillatory condition. The adjustment of contact 228 is preferably such that it cannot be made simply by an operator just as in the previously described embodiment.

The two adjustments result in the adjustment of the amplifier so that excessive feedback does not occur when the unit is conditioned for reception at low volume and transmission at high volume, or for transmission at low volume and reception at high volume; likewise, by making the adjustments with the manual volume control 214 at its maximum volume position, the operator is prevented from adjusting the volume so as to create a condition wherein excessive feedback obtains.

The apparatus at station 2 corresponds with that of station I. The receiver I6 -is connected directly to interstation conductors 304 (the conductors individual to station 2) and the output circuit of amplifier 204, comprising conductors 306, is adapted to be connected to various interstation conductors, including the conductors 212 individual to station I, by means of normally open selector switches 303, 3 I 0 and 3I2. Switches 3I0 and 3I2, like the switches 260 and 262 at station I, are adaptedv to connect the outputof the amplifier tointerstation onductors 3I4 and III associated with the receivers 22 and 24 at sta- .tions 3 and 4. respectively.

.e The interstation conductors 304 are connected 'across a manually operable volume controlv 3I8,.

from which a pair of'. conductors 320 and 322 lead to the loudspeaker I6. The former has in circuit therewith an adjustable resistor 324 corresponding to res tor 280 at station I. Station 2 is also provide with a high volume switch 326, comprising a pair of movable switch blades 328 and 330, normally occupying a position wherein resistor 324 is short-circuited through conductors 332 and 334 and operable into a displaced position wherein the short circuit is removed from around resistor 324, a resistor 338 is placed in parallel with thev receiver, and the feedback circuit of the amplifier 204 is grounded through conductor 338. switch blade 330, and ground connection 340. The feedback circuit of amplifier 204 'is similar in all respects to that of amplifier 202, and consequently the elements thereof have been indicatedV by adding primes to the reference characters.

Station 2 is also provided with a test feedback switch, indicated by reference character 342 and connected and operable in a manner identical with the switch 294 at station I.

The apparatus at station 2 is adjusted for operation in the same manner as that of station I so that it is not deemed necessary to repeat the description of the mode of adjustment.

In the description of the operation of the embodiment of Fig. 3, it is assumed that the operator at station I desires to call an operator at station 2. The operator at station I depresses selector switch 258 to connect the output of amplifier 202 to loudspeaker I6, which is conditioned for high volume reception because of the position of switch 326. The operator at station I may cal1 the operator at station 2 at low volume simply by energizing the amplier 202 by operating the combined volume and power switch 240, and talking into the microphone. If the operator at station I knows that station 2 is located at a relatively noisy place or for any other reason desires to call the operator at station 2 at high volume. he can do so by depressing the high volume switch 282 from its normal to its displaced position. Under normal conditions there is no danger of excessive feedback at this time because the amplifier at station 2 is inoperative so that there'is no feedback path.

When the operator at station 2 responds to the call, he closes selector switch 303, thereby connecting the output of amplifier 204 directly to the receiver I4 at station I and connects the amplifier to its source of power. The two operators can talk to each other at low volume and excessive feedback will not take place because of the previously described adjustment of the units at both stations. If the operator at station 2 has difficulty in hearing, he may so notify the operator at station I, and the latter'needs only to depress the high volume switch `to boostthe amplifier gain and decrease the listening volurnev at station I. As a result'of this operation, the

operator at station 2 is better able to hear the operator at station I. As previously indicated, the high volumeswitch boosts talking volume and decreases listening Avolurn'e in the same relationship so that excessive feedback willnot takev place. Highvolume alternate two-way communication may be carried on by` the operatorslalterwhen they talk and releasing them` when listen.

The utilization of the" inverse feedback in the amplifiers results in many advantageous features in the operation of the system. In the rst place, it is effective to decrease the gain of the ampliiler when acoustic resonance obtains in the loudspeaker, which resonance may occur at a low frequency corresponding to the natural resonance frequency of the loudspeaker. At such a` time the loudspeaker diaphragm moves the-voice coil and this has the effect of increasing the impedance connected across the output of the ampliiier, which in turn has the eifect of increasing the voltage across the windings of the ampliiler output transformer. As a result, the increased voltage across the inverse feedback transformer winding, for instance, the winding 254 at station i, is effective to increase the amount of the inverse feedback with the result that the amplifier gain is decreased. AIn substance, the effect is acoustical smoothing of the loudspeaker response at low frequencies which, in turn, serves l to reduce feedback. This acoustical smoothing of the loudspeaker response at low frequencies and the use of the inverse feedback in the amplifiers for the purpose of cutting down distortion and improving the frequency characteristics of c the ampliners are of considerable benefit in systems of the type herein disclosed, as they increase the volume at which the systems may be operated before feedback occurs.

. A further advantage of the system described above resides in the feature of utilizing the feedback circuit to provide a volume control. This, it may be remembered, is obtained by ysing a movable contact 22B in connection with the feedback and volume control resistor 226.

The arrangement of the last-described embodiment possesses the further advantage that the amplier gain is automatically increased when a pluralityof loudspeakers are connected simultaneously to the amplifier output, as, for instance, when one station calls a number of stations at the same time, as in a paging operation. When more than one speaker is connected to the amplier output. the net result is a decrease in the amplifier output voltage because of the decreased impedance to which the amplifier output is connected. Consequently, the voltage induced in the feedback winding 254 is decreased and the ampliiier gain, in turn, is increased.

A further modification, combining the advantageous features of the embodiment of Fig. 3 with those of the embodiment illustrated in Fig. 1, is illustrated in Fig. 4. Referring now tothis iigure, it may be seenthat the high volume switch comprises three movable switch blades, 350, 352, and 354. These blades are adapted, respectively, to control the loudspeaker circuit, a volume control resistor 358 corresponding to the resistor 56 of the amplifier I8 of the embodiment of Fig. 1,

' and a feedback circuit 358 similar to that shown in Fig. 3. In the normal position oi' the switch,

acaso? g nately depressing their high' volume switches they the feedback circuit and resistor 356 are effective to decrease the gain of the amplier. When the switch is operated to its displaced position, the volume control resistor 35i is short-circuited by switch blade 252 and the feedback circuit 358 is connected to ground through a resistor 360 (which may have a resistance of about 3,000 ohms) and a ground connection 382.

It may be seen from the foregoing that the operation ofthe high volume switch is effective to increase the ampliiier gain, but at the same time to maintain partially eil'ective thefeedback circuit in order to make use of the above enumerated advantages thereof.

'I'he above-described embodiments of the present invention vhave been chosen for the purpose of describing the invention, an'd they are nowise to be construed as limiting the invention in any Way.

What I claim4 is new, and desire to secure by United States Letters Patent is:

:ranged to limit the amplifier gain below excessive feedback point when the microphone is connected to the input and the loudspeaker is teml porarily 'connected to the output of said amplifier and prior to normal use ofthe station in the system. I

2. A multiple unit intercommunication system comprising a number of stations, each having an amplifier with input and output connections, a microphone connected to the input, and a loudspeaker adapted tobe connected to the output of ampliers at other stations in the system, wherein each amplifier is provided with relatively inaccessible amplier gain control means permanently adjusted Just below excessive feedback point when its output is temporarily connected to the loudspeaker at the station and prior tonormal use. of the station in the system.

3. A multiple unit intercommunication system comprising a number of similar stations, each having an amplifier with input and output connections, a microphone, a loudspeaker, and manually operable volume control means associated with the and maximum reception volume positions, each amplifier being adjusted below excessive feedback point when the microphone is connected to the input and the loudspeaker is connected to the output of said amplifier through said volume control means and the volume control means is in its maximum volume position.

4. A multiple unit intercommunication system comprising a plurality of similar stations, each having an ampliiier with input and output connections, a microphone connected to the input, a loudspeaker adapted to 4be connected to the outputs of amplifiers at other stations and having volume decreasing means associated therewith, and means including a high volume switch for substantially simultaneously increasing the amplication and reducing the volume of reception, the amplifier at each station being adjusted below excessive feedback point when its output is connected to the loudspeaker thereat and the volume decreasing means being adjusted below excessive feedback point when the high volume switch is operated with the output of said amplifier connected to the loudspeaker.

5. A multiple unit intercommunication system comprising a plurality of similar stations, each having an amplifier without input and output connections, a microphone connected to the input, a loudspeaker adapted to be connected to i the outputs of ampliiiers at other stations and loudspeaker operable between minimum l vhaving an amplifier with input and output coniication and reduce the volume of reception, and a manually operable reception volume control means operable between minimum and maximum volume positions, the amplifier at each station being adjusted below excessive feedback point when its output is connected to the loudspeaker thereat and theV volume decreasing means being adjusted below excessive feedback point when the high volume switch is operated with the output of said amplifier connected to the loud-K speaker, both said adjustments being made with\ said volume control means in its maximum volume position.

6. A multiple unit intercommunication system comprising a plurality of similar stations, each having an amplifier with input and output connections, a microphone connected to the input, interstation conductors leading therefrom to other stations, a loudspeaker connected to certain of said interstation conductors individual to that station, selector means for connecting the output of the amplifier to interstation conductors individual to other stations in the system, a manual volume control operable between minimum and maximum volume positions interposed between the loudspeaker and the interstation conductors, and means including a manually operable switch adapted substantially simultaneously to increase the amplification and reduce the volume of reception.

'1. A multiple unit intercommunication system u comprising a plurality of similar stations, each having an amplifier with input and output connections, a microphone connected to the input,

interstation conductors leading therefrom to other stations, a. loudspeaker connected to certain of said interstation conductors individual to that station, selector means for connecting the output of the amplifier to interstation conductors individual to other stations in the system, a manual volume control operable between minimum and maximum volume positions interposed between the loudspeaker and the interstation conductors, reception volume adjusting means interposed between the loudspeaker and said volume control means, amplifier gain control means, and means including substantially simultaneously operable switches for increasing the amplification and reducing the volume of reception in the same relationship.

8. A multiple unit intercommunication system 'J comprising a plurality of similar stations, each having an amplifier with input and output connections, a microphone connected to the input, interstation conductors leading therefrom to other stations, a loudspeaker connected to certain of said interstation conductors individual to that station, selector means for connecting the output of the amplifier to interstation conductors individual to other stations in the system, a manual volume control operable between minimum and maximum volume positions interposed between the loudspeaker and the interstation conductors, a normally ineffective volume reducing means interposed between the loudspeaker and said volume control means, normally ineffective amplification increasing means, saidivolume reducing and amplification increasing means being adjusted to decrease the receiver volume and increase the amplification inthe same relationship, and means for substantially simultaneously rendering effective said volume reducing means and ineffective said amplification reducing means.

9. A multiple unit intercommunication system comprising a plurality of similar stations, each nections, a microphone `onnected to the input, interstation conductors leading therefrom to other stations, a loudspeaker connected to certain of said interstation conductors individual to that station, selector means for connecting the output of the amplifier to interstation conductors individual to other stations inthe system, a manual volume control operable between *minimum and maximum volume positions interposed between the loudspeaker and the interstation conductors, normally ineffective resistance means interposed between the loudspeaker land said volume control means for lreducing `the loudspeaker reception volume, resistance means in circuit with the amplier normally effective to reducethe amplification, said resistance means being adjusted to exert substantially the same effect upon the reception and transmission volumes, and meansincluding a manually operable switch adapted substantially simultaneously to place the first-mentioned resistance means in circuit with the loudspeaker and to short circuit the second-mentioned resistance means.

10. The method of adjusting a communication unit comprising an amplifier having amplification control means,.a microphone connected thereto, and a loudspeaker adapted to be supplied with amplified voice currents from amplifiers at other similar stations, which includes the step of temporarily connecting the output of the amplifier to the loudspeaker and adjusting the amplification fixedly to a point just below excessive feedback prior to normal use of the unit in a communication system.

11. The method of adjusting a communication unit comprising an amplifier conditioned to transmit at normal and at high volume and having amplification control means, a microphone connected thereto, and a loudspeaker conditioned to receive at normal or low volume and having associated therewith volume control means adapted to be supplied with amplified voice currents from amplifiers at other similar stations, which includes connecting the output of the amplifier conditioned for transmission at low volume to the receiver conditioned for reception at normal volume and first adjusting the amplication to a point just below excessive feedback and then conditioning the amplifier for transmission at high volume and the receiver for reception at low volume and adjusting the receiver reception volume to a point just below that at which excessive feedback occurs.

12. A unit for use in an intercommunication system, including in combination, an amplifier, a microphone, a loudspeaker, and amplifier gain control means associated with said amplifier, the gain control means being arranged to limit the amplifier gain to a point below that at which excessive feedback occurs when the microphone is connected to the input and the loudspeaker is temporarily connected to the output of said amplifier prior to normal use of the unit in the sys- 13. A unit for use in an intercommunication system, including in combination, an amplifier conditioned to amplify a predetermined amount having input and output connections, a microphone directly connected to the input, selector switch means connected to the output for selectively connecting the output to-the 'receivers of similar units, a loudspeaker adapted to be energized by amplifiers of similar units and conditioned to rece1ve at al predetermined volume,

means operable to increase the amplification of said amplifier, and means operable to decrease the reception volume of said receiver, said two last-mentioned means being so arranged that the amplification is increased in the same relation as the receiver volume is reduced.

1li. A unit for use in an intercommunication system, including in combination, an amplifier normally conditioned to amplify at a normal value, means associated with said amplifier for increasing the amplification. a microphone, a

loudspeaker conditioned for reception at a normal volume, means associated with said loudspeaker for decreasing` the volume of reception,

said amplifier being adjusted to a point 4below that at which excessive feedback occurs when the microphone is connected to the input and the loudspeaker is connected to the output of said amplifier and the amplifier andreceiver are cons ditioned for normal amplification and reception, and said loudspeaker volume being adjusted to a point below that at which excessive feedback ocy curs when the microphone and loudspeaker are point below excessive feedback when the microphone is connected to the input and the loudspeaker is connected to the output of said amplifier through said volume control means and with said control means in its maximum volume position.

16.- A unit for use in an intercommunication system, including in combination, an amplier, a microphone,a loudspeaker, and means including a high volume switch for simultaneously increasing the amplification and decreasing the volume of reception, the amplification of said amplier being adjusted to a point just below excessive feedback when the loudspeaker is connected to its output, and said volume decreasing means being adjusted to a point just below excessive feedback when the high volume switch is operated with the loudspeaker connected to the output of the amplifier.

17. A unit for use in an intercommunication system, including in combination, an amplifier having amplication adjusting means, a microphone, a loudspeaker having reception volume adjusting means. an interstation communication circuit individual to the unit connected to the loudspeaker, normally operable loudspeaker volume control means in said circuit operable between minimum and maximum volume positions, a high volume switch and means controlled thereby for simultaneously increasing the volume of reception and decreasing the amplification, the amplification of said amplifier being adjusted to a point below excessive feedback when the interstation circuitis connected to the output of said amplifier and the loudspeaker reception volume being adjusted to a point below excessive feedback when the high volume switch is operated with the output of said amplifier connected to the interstation circuit, both said adjustments being made with the volume adjusting means in its maximum volume position.

asaaao'r 18. A unit for use in an intercommunication system, including in combination, an amplifier having input and output connections, a microphone connected to the input, .a loudspeaker adapted to be supplied with amplified voice currents from other stations, means i'or increasing the amplification to condition the ampliierfor transmission at high volume, means for reducing the volume of reception to condition said loudspeaker for reception at low volume. said two last-mentioned means being operable substantially simultaneously, means associated with said amplifier for adjusting the amplification, means associated.with said receiver for adjusting the reception volume, means for connecting the output of said amplifier to said loudspeaker, said amplification adjusting means being adjusted to a point just below excessive feedback when the output is connected to said loudspeaker and said amplifier and receiver are conditioned for transmission at low volume and reception at high volume, respectively, and said volume adjusting means being adjusted to a point just below excessive feedback when the output is connected to the receiver and the amplifier is conditioned for high volume transmission and the receiver is conditioned for low volume reception.

19. In apparatus of the type described, the combination including, a loudspeaker, an amplifier, and means including switches for decreasing the loudspeaker reception volume and increasing the gain of the amplifier, said switches being so constructed and arranged that the loudspeaker reception volume is decreased before the amplier gain is increased.

20. In apparatus of the type described, the combination including, a loudspeaker, an amplifier, loudspeaker reception volume control means, amplifier gain control means, and means including switches normally conditioning the loudspeaker for reception at high volume and the amplifier for transmission at low volume and simultaneously operable to condition the loudspeaker for reception at low volume and the amplifier for transmission at high volume, said switches and control means being so constructed and arranged that the reception Volume is decreased before the amplifier gain is increased and the amplifier gain decreased before the reception volume is increased.

21. A multiple unit intercommunication system comprising a plurality of similar stations, each having an amplifier with input connections and an output transformer having a pair of secondary windings, a microphone connected to the input connections, interstation conductors leading therefrom to other stations, a loudspeaker connected to certain of said interstation conductors individual to that station, selector means for connecting one of the secondary windings of said output transformer to interstation conductors individual to other stations in the system, a manual volume control operable between minimum and maximum volume positions interposed between the loudspeaker and its associated interstation conductors, normally ineffective resistance means for decreasing the loudspeaker reception volume, means including an inverse feedback circuit connected to the other of said output transformer secondary windings normally effective to reduce the gain of the amplifier, said resistance means and last-mentioned means being adjusted to exert substantially the same effect upon the reception and amplifier transmission volumes, and means including a manually operable switch adapted substantially simultaneously to render effective the reception volume decreasing means and to render ineffective the inverse feedback circuit.

22. A multiple unit intercommunication system comprising a plurality of similar stations, each having an amplifier provided with input connections and an output transformer comprising a pair of secondary windings, a microphone connected to the input connections, interstation conductors leading therefrom to the other stations, a loudspeaker connected to certain of said interstation conductors individual to that station, selector means for connecting one of the secondary windings of the output transformer to interstation conductors individual to other stations in the system, a manual volume control operable between minimum and maximum volume positions interposed between the loudspeaker and the interstation conductors, normally ineffective resistance means for decreasing the loudspeaker reception volume, resistance means normally effective to reduce the gain of the amplifier, means including an `inverse feedback circuit connected to the other of the output transformer secondary windings normally effective to reduce the gain of the amplifier, said mst-mentioned resistance means and the second-mentioned resistance means and inverse feedback circuit being adjusted to exert substantially the same effect upon the reception and transmission volumes, and means comprising a manually operable switch adapted substantially simultaneously to render effective said rst-mentioned resistance means to decrease the loudspeaker reception volume, and to short circuit said second-mentioned.resistance means and to decrease the amount of feedback thereby to increase the gain of the amplifier.

23. A unit for use in an intercommunication system, including in combination, an amplier. a loudspeaker, resistance means in circuit with the amplifier normally effective to reduce the amplifier gain, normally ineffective resistance means adapted to be placed in circuit with the loudspeaker for reducing the reception volume, and means including a pair of manually operable switches for substantially simultaneously short circuiting said first-mentioned resistance means to increase the amplifier gain and for rendering effective said second-mentioned resistance means to decrease the loudspeaker reception volume.

24. A unit for use in an intercommunication system, including in combination, an amplifier, a loudspeaker, means including an inverse feedback connection normally effective to reduce the amplifier gain, normally ineffective resistance means adapted to be placed in circuit with the loudspeaker for reducing the reception volume, and means including a pair of manually operable switches for substantially simultaneously grounding said feedback connection to increase the amplifier gain and for rendering effective said second-mentioned resistance means to decrease the loudspeaker reception volume.

25. A unit for use in Aan intercommunication system, including in combination, an amplifier, a loudspeaker, resistance means in circuit with the amplifier normally effective to reduce the amplifier gain, means including an inverse feedback connection normally effective to reduce the amplifier gain, normally ineffective resistance means adapted to be placed in circuit with the loudspeaker for reducing the reception volume, a resistor, and means including three manually operable switches for substantially simultaneously short circuiting said first-mentioned resistance means and grounding the feedback circuit through said resistor to increase the amplifier gain and for rendering effective said second-mentioned resistance means to decrease the loudspeaker reception volume.

- RICHARD H. CAMPBELL. 

